Heretofore, for the material of the light-transmissible finger touch covers for illuminated switches mentioned above, silicone rubber, exclusively has found practical use due to its light permeability and excellence in the heat resistance, weatherability and electric isolation performance.
For the inks to be printed on such finger touch cover, they have been limited only to those having film-forming components based on silicone because of their better compatibility with the silicone rubber for attaining a firm fixation on the cover. Such inks should reveal a high light-blocking ability, in order to attain a high relief contrast of the illuminated pattern and, therefore, are pigmented usually in black or dark gray color. Usually, carbon black is employed here as the coloring pigment in an amount of 10-20% or even more, based on the weight of the film-forming component (base polymer) of the ink.
While there have been used, in practice, inks based on silicone for printing the light-transmissible finger touch covers, as mentioned above, such inks have disadvantages of inferior wear-resistance and lower oil-resistant property. In particular, in case of using carbon black for obtaining black or dark gray colored ink, the amount of the base polymer per unit surface area of the printing layer is reduced correspondingly, resulting in a reduction in the wear-resistance so that the print may easily be worn out by rubbing with a finger upon repeated finger touch on the switch or by contact with other materials, such as clothes, etc. Moreover, the highly stuffed carbon black may easily be rubbed off and stain the finger.
Under these circumstances, employment of other rubber materials, including an ethylene-propylene rubber (EPDM), which also exhibit a superior transparency, for the material of finger touch cover may be considered. Thus, attempts had been made for realizing such possibility. However, no satisfactory durability of printing layer with any ink was attained.
As another approach for solving these problems, the inventors have attempted to develop a novel modified material for the finger touch cover of which material property is improved such that the bonding between the ink printing layer and the substrate finger touch cover made thereof is high enough to insure a high durability of the printing layer. The inventors have discovered that a multicomponent rubber material based on EPDM, combined with another elastomeric component based on butadiene rubber, reveals a high bonding performance to printing layer of various inks.